Note:
I have been unable to get libpng capturing to work under Cygwin (so disable HAVE_PNG in the makefile).
I would appreciate any tips if you get this working.
Note:
You may need libpng in Mac OS X to save images.
Ethan Tira-Thompson has an OS X port of libpng here.

Double left-click to place a black stone; double right-click to place a white stone.

Left- and right- arrow keys undo and redo moves (you can watch the whole game after it's done).

SPACEBAR toggles the brakes, which are on by default. Letting the board float around with no brakes is a good way to find moves (remember, this is projective 3-space).

The 'r' key randomly plays a stone.
Holding the 'r' key down will play an entire random game (and then some).

The 'p' key pauses the game, and the 'f' key switches to (but not from) fullscreen.

The ESCAPE key exits.

Rules:
Captured stones are automatically removed from the board.
Jenn-Go uses Chinese scoring, so you have to fill in all but your two eyes in each group to get points.
(The the motivation for Chinese scoring is in simplicity of scoring algorithm.
I have yet to sucessfully explain Japanese scoring to a beginner.)

Choosing a board
This has to be done at the command line. Jenn-Go inputs the six elements of the upper-triangle of a Coxeter matrix and then any subset of the numbers [0-3] deonting which generators are included in the vertex symmetry group.
Some nice boards are listed in jennhelp.txt. Also try